Is it in the correct folder?
Your Home folder
Library
Mail
Bundles
> 1. I supposed that the installation fixer would delete some file(s) related to previous versions of MailGrowl.
Not exactly; the Fixer deletes the installation receipt, which is an entry in a database, not a separate file.
> It seems that the receipt lurks somewhere …
Since the Fixer changed the installation result from failure to success, I would infer that you did have the problem the Fixer addresses, and the Fixer worked successfully, and now you have (or already had but had not yet reached) a different problem.
> 2. Is it possible to get a copy of GrowlMail 1.2.3?
Yes, but let's see if we can get 1.2.4 working for you.
Next question: If you run the following two commands in Terminal, what is the output?
defaults read com.apple.mail EnableBundles
defaults read com.apple.mail BundleCompatibilityVersion
Copy and paste both lines from here into the terminal window.